The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to the detection of wide bandwidth noise in NMR signals acquired during a scan and the elimination of artifacts produced by such noise in the reconstructed image.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field b.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A radio-frequency (RF), which is also denoted the NMR signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated and this signal may be received and processed to form an image.
This "NMR" signal is very small and extraordinary measures are taken to shield the MRI system from external RF noise and to eliminate sources of internal noise. Nevertheless, generators of short-duration noise pulses persist and may elude location and elimination. These noise pulses are referred to as "spike noise", "impulse noise" or "white pixels", and lead to image artifacts with such vernacular names as corduroy and zebra artifacts. Sources of such noise include arcing due to partial discharges from intermittent electrical contacts or electrostatic discharge, and harmonics of fast transients such as those caused by ground loops. When such noise sources occur regularly, their source can be located and measures can be taken to eliminate them. This "hardening" process occurs at any new MRI installation, and eventually all the short-duration noise sources are eliminated except those which are intermittent and defy cost-effective diagnosis.
A number of strategies have been employed to mitigate the effects of intermittent noise sources. Such methods include the examination of the acquired NMR signals to locate noise spikes or the examination of the reconstructed image to locate the effects of such noise. These prior methods work when the noise spike occurs in NMR signals that are heavily phase or frequency encoded (i.e. on the edges of k-space), but they do not perform well when the noise spike occurs in NMR signals near the center of k-space. In the latter case the NMR signal magnitude is quite large and it is more difficult to discern signal from noise. Noise spikes detected by such methods are typically removed by interpolating between the adjacent values.